1. Field of the Invention
The invention relates to the field of methods for synthesizing an intervisibility image consisting of pixels and representing the distribution, over a terrain overflown by an aircraft, of the intervisibility zones. The terrain overflown by the aircraft is either the terrain actually overflown by the aircraft when the image is displayed, or the terrain which is intended to be overflown by the aircraft, the image then being displayed in anticipation.
As depicted in FIG. 1, the intervisibility zones may be zones of indirect intervisibility between the aircraft and at least one given potential threat, that is to say places from where the aircraft is visible to at least one threat. The potential threat is said to be given since it is generally known and held in the database of the cartographic system implementing the relevant method of synthesizing an intervisibility image. With this type of threat is also associated a range zone which represents the theoretical maximum range of the threat. The intervisibility zone is equal to the range zone or strictly included within the range zone, depending on whether there exist various obstacles, such as a mountain, for example, that reduce the theoretical range zone, the intervisibility zone corresponding to the actual range zone in a given real situation with regard to a given aircraft. Specifically, the intervisibility zone often depends on the altitude of the aircraft. Unless otherwise stated in the text, the range zone is understood to mean the theoretical maximum range zone as opposed to the actual range zone related to the presence of obstacles which actual range zone corresponds to the intervisibility zone.
The intervisibility zones may also be zones of direct intervisibility between the aircraft and at least one potential threat, that is to say places visible in respect of the aircraft. The invisible places in respect of the aircraft are liable to conceal a threat. On the other hand, here the threat is a generally purely hypothetical threat, insofar as there may not be any actual threat at places which are invisible in respect of the aircraft. In the method of synthesizing an intervisibility image according to the invention, the distribution of these intervisibility zones is preferably considered only in a given part of the map displayed on the screen of the aircraft, corresponding to a sector of visibility of the aircraft, and not over the whole of this map.
The method of synthesizing an intervisibility image is preferably carried out by certain functional blocks of a cartographic function of a cartographic accelerator card described in detail subsequently and using the data of the aforesaid database. The information cues contained in the intervisibility image are advantageously intended to be combined with information cues contained in other images such as a forest image or an anticollision or hypsometric coloration image for example so as to generate an altimetric image which is itself used for the generation of a generally 2D5 cartographic image of the terrain overflown by the aircraft, that is to say representing a plan view of the terrain which is modulated, pixel by pixel, by a shading cue representative of the relief of the terrain.
2. Description of the Prior Art
According to a prior art, the intervisibility zones are represented on the map displayed by a so-called “ray tracing” technique represented in particular in American patent U.S. Pat. No. 5,086,396. This technique is limited to the case of indirect intervisibility. Rays are traced from the position of the threat considered up to either an obstacle limiting the actual range of the threat, or the limit of the theoretical range in case of absence of obstacle. The result is visible in FIG. 7, of said American patent. The intervisibility zone corresponds to the zone covered by the rays. The theoretical range of the threat is indicated by a generally circular line 23A. In a preferred option, at the level of claim 7, said American patent proposes that a different color be assigned to each family of rays representing a different threat type, so as undoubtedly to allow the pilot to distinguish between the various types of threat. This grid of rays traced from the threat is added onto the coloration of the map displayed. This prior art has several drawbacks. Thus, this prior art causes the loss of certain information cues, such as the shading cue representing the relief of the terrain, for example, in the parts of the map which are covered by rays. For the parts of the map which are in the zones of intervisibility of several threats, possibly of differing type, the reading of the cues conveyed by the map under the interleaving of the rays becomes awkward or even impossible for the pilot. Moreover, the range zone is indicated only by its limit, thus sometimes preventing the pilot from viewing it properly in its entirety. Finally, the focus of the rays departs from the position of the threat, thereby preventing the use of this technique in the case of searching for zones of direct intervisibility where the potential threats are not located.